Préparation de réseaux organiques covalents monocristallins par polymérisation de composés polynitroso aromatiques.

La construction modulaire est une stratégie émergente qui permet la fabrication de matériaux ordonnés à l’échelle atomique. Elle consiste en l’association programmée de sous-unités moléculaires via des sites réactifs judicieusement sélectionnés. L’application de cette stratégie a d’ores et déjà produit des matériaux aux propriétés remarquables, notamment les réseaux organiques covalents, dans lesquels des atomes de carbone et d’autres éléments légers sont liés de manière covalente. Bien que des matériaux assemblés par des interactions non-covalentes puissent être préparés sous la forme de monocristaux macroscopiques de cette façon, ceci n’était pas possible dans le cas des réseaux organiques covalents. Afin de pallier cette lacune, nous avons choisi d’étudier des réactions de polymérisation réversibles ayant lieu par un mécanisme d’addition. En effet, l’hypothèse de départ de cette thèse suppose qu’un tel processus émule le phénomène de cristallisation classique – régi par des interactions non-covalentes – et favorise la formation de monocristaux de dimensions importantes. Pour tester la validité de cette hypothèse, nous avons choisi d’étudier la polymérisation des composés polynitroso aromatiques puisque la dimérisation des nitrosoarènes est réversible et procède par addition. Dans un premier temps, nous avons revu en profondeur la littérature portant sur la dimérisation des nitrosoarènes. À partir des données alors recueillies, nous avons conçu, dans un deuxième temps, une série de composés polynitroso ayant le potentiel de former des réseaux organiques covalents bi- et tridimensionnels. Les paramètres thermodynamiques propres à leur polymérisation ont pu être estimés grâce à l’étude de composés mononitroso modèles. Dans un troisième temps, nous avons synthétisé les divers composés polynitroso visés par notre étude. Pour y parvenir, nous avons eu à développer une nouvelle méthodologie de synthèse des poly(N-arylhydroxylamines) – les précurseurs directs aux composés polynitroso. Dans un quatrième temps, nous avons étudié la polymérisation des composés polynitroso. En dépit de difficultés d’ordre pratique causées par la polymérisation spontanée de ces composés, nous avons pu identifier les conditions propices à leur polymérisation en réseaux organiques covalents hautement cristallins. Plusieurs nouveaux réseaux covalents tridimensionnels ont ainsi été produits sous la forme de monocristaux de dimensions variant entre 30 µm et 500 µm, confirmant la validité de notre hypothèse de départ. Il a par conséquent été possible de résoudre la structure de ces cristaux par diffraction de rayons X sur monocristal, ce qui n’avait jamais été possible dans le passé pour ce genre de matériau. Ces cristaux sont remarquablement uniformes et les polymères qui les composent ont des masses moléculaires extrêmement élevées (1014-1017 g/mol). Toutefois, la polymérisation de la majorité des composés polynitroso étudiés a plutôt conduit à des solides amorphes ou à des solides cristallins constitués de la forme monomérique de ces composés. D’autres composés nitroso modèles ont alors été préparés afin d’expliquer ce comportement, et des hypothèses ont été émises à partir des données alors recueillies. Enfin, les structures de plusieurs composés polynitroso ayant cristallisés sous une forme monomérique ont été analysés en détails par diffraction des rayons X. Notre stratégie, qui consiste en l’utilisation de monomères ayant la capacité de polymériser spontanément par un processus d’addition réversible, semble donc prometteuse pour obtenir de nouveaux réseaux covalents monocristallins à partir de composés polynitroso ou d’autres monomères de nature similaire. De plus, les résultats présentés au cours de cette thèse établissent un lien entre la science des polymères et la chimie supramoléculaire, en illustrant comment des structures ordonnées, covalentes ou non covalentes, peuvent toutes deux être construites de façon prévisible.

Synthesis, characterization and hydrodesulphurisation activity of CoMo/gama-Al2O3 catalyst prepared through molecular designed dispersion method

CoMo/gama-Al2O3 catalysts for hydrodesulphurisation activity were prepared by making use of the molecular designed dispersion (MDD) method. Molybdenum and cobalt pyrrolidine-N-carbodithioate (Pydtc) complexes were used for the incorporation of metals on the support. The catalysts were characterized by elemental analysis, low temperature oxygen chemisorption, temperature programmed reduction (TPR) and laser Raman spectroscopy. The hydrodesulphurisation activity of all the catalysts were carried out and results were compared with those of the catalysts prepared through the conventional method. Higher molybdenum dispersion, smaller molybdenum clusters, lower reduction temperature of catalyst and better hydrodesulphurisation activity were observed for the catalysts prepared through the MDD method

Acidity and catalytic activity of rare earth modified Al/Zr pillared clays

The pillared montmorillonite has been prepared by exchanging Na+ in the interlayer of montmorillonite with Al hydroxy cation followed by calcination. Pillared clays are also prepared after exchanging Na' ions with Ce, La, Sm-ions and then pillarcd with aluminium oxides, The surface characterisation of the prepared catalysts has been done using XRD and surface area measurements. To probe the acidic property of the system, temperature programmed desorption (TPD) of NH, has been done. Toluene alkylation by benzyl chloride has been carried out for the evaluation of catalytic activity. The most active system is found to be mixed Al/Zr pillarcd montrnorillonite.

Effect of vanadia on physico-chemical and catalytic characteristics of dysprosia

Physico-chemical characterization of DY203/V2O5 systems prepared through wet impregnation method has been carried out using various techniques like EDX, XRD, FTIR. thermal studies, BET surface area, pore volume and pore size distribution analysis. The amount of vanadia incorporated has been found to influence the surface properties of dysprosia. The spectroscopic results combining with X-ray analysis reveal that vanadia species exist predominantly as isolated amorphous vanadyl units along with crystalline dysprosium orthovanadate. Basicity studies have been conducted by adsorption of electron acceptors and acidity and acid strength distribution by temperature programmed desorption of ammonia. Cyclohexanol decomposition has been employed as a chemical probe reaction to examine the effect of vanadia on the acid base property of Dy2O3. Incorporation of vanadia titrates thc Lewis acid and base sites of Dy2O3, while an enhancement of Bronsted acid sites has been noticed. Data have been correlated with the catalytic activity of these oxides towards the vapour phase methylation of phenol

Surface acidity and catalytic activity of mixed Fe-Al and Cr-Al pillared montmorillonites

The incorporation of transition metal oxide pillars such as those of iron and chromium along with Al2O3, pillars within the interlayers of a montmorillonite clay is investigated. The surface acidity of these catalysts has been evaluated for the first time employing the equilibrium adsorption of an electron donor, perylene, and the results are compared with those obtained by temperature programmed desorption of ammonia. The principle is based on the ability of a catalyst surface site to accept a single electron from an electron donor like perylene to form charge transfer complexes and the amount of adsorbed species is measured quantitatively by UV-vis spectroscopy. Fina1ly, an attempt has been made to correlate the acidity determined by the two independcnt methods and the catalytic activity of present systems in the benzoylation of toluene with benzoyl chloride. Incorporation of Fe and Cr has changed the properties of AI pitlared montmorillonite. Fe pillared systems have been found to be vcry good catalysts for benzoylation reaction

Synthesis, characterisation and benzylation activity of vanadia impregnated iron pillared montmorillonite

Iron pillared Montmorillonite has been synthesised and it is then wet impregnated with vanadia with different vanadia composition. These catalysts are characterised using conventional techniques such as XRD analysis,FTIR analysis and surface area and pore volume measurements. Acidity is measured using spectrophotometric monitoring of adsorption of perylene, thermogravimetric desorption of 2.6 dimethylpyridine and temperature programmed desorption of ammonia. Activity studies are done in the liquid phase. It has been concluded that Lewis acidic sites are responsible for the benzylation of toluene when the benzylating agent is benzyl chloride while Bronsted acidic sites are responsible when the reagent is benzyl alcohol.

Liquid phase benzylation of o-xylene over pillared clays

Iron, aluminium and mixed iron aluminium pillared clays have been prepared by partial hydrolysis method and doped with IO% Mo, V and Cr. The samples have been characterised by XRD, FTIR and surface area and pore Volume measurements. The surface acid site distribution has been determined by temperature programmed desorption of ammonia. Vanadia incorporated systems show maximum acidity. Benzylation of o-xylene has been done as probe reaction to test catalytic activity. Benzyl chloride is a superior benzylating agent compared to benzyl alcohol in activity and selectivity. Cent percent selectivity towards monobenzylated product is obtained in all the cases. Fe pillared systems exhibit maximum activity. The catalytic activities of the systems can be correlated with the amount of strong add sites. The effects of various reaction variables on the reaction have been studied. Presence of moisture has a diminishing effect on the reaction rate.

Selective Methyl Tert-Butyl Ether Synthesis over Metal Exchanged Pillared Clays

Iron and mixed iron aluminium pillared montmorillonites prepared by partial hydrolysis method was subjected to room temperature exchange with transition metals of the first series. The materials exhibit good structural as well as thermal stability. Exchanged metals were found to be present inside the porous network, in the environs of the pillars. Mixed pillaring resulted in the intercalation of Al 13 like polymers in which Al is partially substituted by Fe. The acidic structure was followed by temperature programmed desorption of ammonia and cumene cracking test reaction. Weak and medium sites overshadow the strong sites in all systems. However, exchange with metals increases the number of strong sites. The prepared materials are efficient catalysts for gas phase MTBE synthesis. The catalytic activity can be well correlated with the total amount of weak and medium acid sites.

Liquid phase benzoylation of arenes over iron promoted sulphated zirconia

The present work undertakes the preparation and physico-chemical characterisation of iron promoted sulphated zirconia (SZ) with different amounts of iron loading and their application to Friedel-Crafts benzoylation of benzene, toluene and xylene under different experimental conditions, XRD and laser Raman techniques reveal the stabilisation of the tetragonal phase of zirconia and the existence of iron in highly dispersed form as Fe203 on the catalyst surface. The surface acidic properties were determined by ammonia temperature programmed desorption (TPD) and perylene adsorption, The results were supported by the TGA studies after adsorption of pyridine and 2,6-dimethylpyridine (2,6-DMP), Strong Lewis acid sites on the surface, which are evident from TPD and perylene adsorption studies. explain the high catalytic activity of the systems towards benzoylation. The experimental results provide evidence for the truly heterogeneous nature of the reaction. The studies also establish the resistance to deactivation in the metal incorporated sulphated systems.

Synthesis, characterization and benzylation activity of nanocrystalline chromia loaded sulfated titania prepared via sol–gel route

Chromia loaded sulfated titania has been synthesized via sol–gel route with different chromia loadings. These catalysts are characterized using conventional techniques such as XRD analysis, FTIR analysis, surface area and pore volume measurements, EDX, SEM and UV–Vis diffuse reflectance spectral analysis. Acidity is measured using spectrophotometric monitoring of adsorption of perylene, thermogravimetric desorption of 2,6-dimethylpyridine and temperature programmed desorption of ammonia. Activity studies are done in the liquid phase. It has been concluded that Lewis acid sites are responsible for the benzylation of arenes with benzyl chloride.

Synthesis, characterization and catalytic activity of Nd203 supported V205 catalysts

A series of rare-earth neodymia supported vanadium oxide catalysts with various V205 loadings ranging from 3 to 15 wt.% were prepared by the wet impregnation method using ammonium metavanadate as the vanadium precursor. The nature of vanadia species formed on the support surface is characterized hy a series of different physicochemical techniques like X-ray diffraction (XRD). Fourier transform infrared spectroscopy (FTIR). BET surface area, diffuse reflectance UV-vis spectroscopy (DR UV-vis), thermal analysis (TG-DTG/DTA) and SEM. The acidity of the prepared systems were verified by the stepwise temperature programmed desorprion of ammonia (NH3-TPD) and found that the total acidity gets increased with the percentage of vanadia loading. XRD and FT1R results shows the presence of surface dispersed vanadyl species at lower loadings and the formation of higher vanadate species as the percentage composition of vanadia is increased above 9 wt.%. The low surface area of the support. calcination temperature and the percentage of vanadia loading are found to influence the formation of higher vanadia species. The catalytic activity of the V205-Nd203 catalysts was probed in the liquid phase hydroxylation of phenol and the result show that the present catalysts are active at lower vanadia concentrations.

Influence of surface and acid properties of vanadia supported on ceria promoted with rice husk silica on cyclohexanol decomposition

Cyclohexanol decomposition activity of supported vanadia catalysts is ascribed to the high surface area, total acidity and interaction between supported vanadia and the amorphous support. Among the supported catalysts, the effect of vanadia over various wt% V2O5 (2–10) loading indicates that the catalyst comprising of 6 wt% V2O5 exhibits higher acidity and decomposition activity. Structural characterization of the catalysts has been done by techniques like energy dispersive X-ray analysis, X-ray diffraction and BET surface area. Acidity of the catalysts has been measured by temperature programmed desorption using ammonia as a probe molecule and the results have been correlated with the activity of catalysts.

Catalysis by Transition Metal Modified Ceria and Ceria-Zirconia Mixed Oxides Prepared via Sol-Gel Route

The aim of catalysis research is to apply the catalyst successfully in economically important reactions in an environmentally friendly way. The present work focuses on the modification of structural and surface properties of ceria and ceria-zirconia catalysts by the incorporation of transition metals. The applications of these catalysts in industrially important reactions like ethylbenzene oxidation, alkylation of aromatics are also investigated.Sol-gel method is effective for the preparation of transition metal modified ceria and ceria-zirconia mixed oxide since it produces catalyst with highly dispersed incorporated metal. Unlike that of impregnation method plugging of pores is not prominent for sol-gel derived catalyst materials. This prevents loss of surface area on metal modification as evident for BET surface area measurements.The powder X-ray diffraction analysis confirms the cubic structure of transition metal modified ceria and ceria-zirconia catalysts. The thermal stability is evident from TGA/DTA analysis. DR UV-vis spectra provide information on the coordination environment of the incorporated metal. EPR analysis ofCr, Mn and Cu modified ceria and a ceria-zirconia catalyst reveals the presence of different oxidation states of incorporated metal.Temperature programmed desorption of ammonia and thermogravimetric desorption of 2,6-dimethyl pyridine confirms the enhancement of acidity on metal incorporation. High a-methyl styrene selectivity in cumene cracking reaction implies the presence of comparatively more number of Lewis acid sites with some amount of Bronsted acid sites. The formation of cyclohexanone during cyclohexanol decomposition confirms the presence of basic sites on the catalyst surface.Mn and Cr modified catalysts show better activity towards ethylbenzene oxidation. A redox mechanism through oxometal pathway is suggested.All the catalysts were found to be active towards benzylation of toluene and a-xylene. The selectivity towards monoalkylated products remains almost 100%. The catalytic activity is correlated with the Lewis acidity of the prepared systems.The activity of the catalysts towards methylation of phenols depends on the strength acid sites as well as the redox properties of the catalysts. A strong dependence of methylation activity on the total acidity is illustrated.

RNS based programmable multi-mode decimation filter for WCDMA and WiMAX

The recent trends envisage multi-standard architectures as a promising solution for the future wireless transceivers to attain higher system capacities and data rates. The computationally intensive decimation filter plays an important role in channel selection for multi-mode systems. An efficient reconfigurable implementation is a key to achieve low power consumption. To this end, this paper presents a dual-mode Residue Number System (RNS) based decimation filter which can be programmed for WCDMA and 802.16e standards. Decimation is done using multistage, multirate finite impulse response (FIR) filters. These FIR filters implemented in RNS domain offers high speed because of its carry free operation on smaller residues in parallel channels. Also, the FIR filters exhibit programmability to a selected standard by reconfiguring the hardware architecture. The total area is increased only by 24% to include WiMAX compared to a single mode WCDMA transceiver. In each mode, the unused parts of the overall architecture is powered down and bypassed to attain power saving. The performance of the proposed decimation filter in terms of critical path delay and area are tabulated.

Dual-mode RNS based programmable decimation filter for WCDMA and WLANa

The recent trends envisage multi-standard architectures as a promising solution for the future wireless transceivers. The computationally intensive decimation filter plays an important role in channel selection for multi-mode systems. An efficient reconfigurable implementation is a key to achieve low power consumption. To this end, this paper presents a dual-mode Residue Number System (RNS) based decimation filter which can be programmed for WCDMA and 802.11a standards. Decimation is done using multistage, multirate finite impulse response (FIR) filters. These FIR filters implemented in RNS domain offers high speed because of its carry free operation on smaller residues in parallel channels. Also, the FIR filters exhibit programmability to a selected standard by reconfiguring the hardware architecture. The total area is increased only by 33% to include WLANa compared to a single mode WCDMA transceiver. In each mode, the unused parts of the overall architecture is powered down and bypassed to attain power saving. The performance of the proposed decimation filter in terms of critical path delay and area are tabulated